Aspartame (APM) is one of the most widely used artificial sweeteners in the world. First approved by the U.S. Food and Drug Administration (FDA) for limited use in solid food in 1981, its authorization was extended to soft drinks in 1983 and then approved as a general sweetener in 1996 (FDA 1981, 1983, 1996). Likewise, the sweetener was approved for general use in the European Union in 1994 (EC Directive 1994). APM is now present in > 6,000 consumer packaged goods and in nearly 500 pharmaceutical products, including children’s medicines (Aspartame Information Center 2005). In the United States, > 70% of aspartame sales are attributed to soft drinks (American Dietetic Association 2004). The acceptable daily intake (ADI) of aspartame is currently 50 mg/kg body weight (bw) in the United States and 40 mg/kg bw in the European Union for both children and adults. Daily consumption of artificial sweeteners by women of childbearing age and by children has been estimated at 2.5–5.0 mg/kg bw (Butchko et al. 2002). In a study of Swedish diabetics, the general APM intake was lower than the ADI, but the worst-case calculation of intake in the children’s group was 114% of the ADI (Ilback et al. 2003). APM is metabolized in the gastric tract of rodents, nonhuman primates, and humans to its three constituents: aspartic acid, phenylalanine, and methanol. When absorbed, aspartic acid is transformed into alanine plus oxaloacetate (Stegink 1984); phenylalanine is transformed mainly into tyrosine and, to a lesser extent, phenylethylamine and phenyl-pyruvate (Harper 1984); and methanol is transformed into formaldehyde and then to formic acid (Opperman 1984). In vitro and in vivo tests demonstrate that APM is not genotoxic. Likewise, long-term carcinogenicity studies conducted by the manufacturers of aspartame using rats and mice in the 1970s and 1980s did not demonstrate any carcinogenic effects. A detailed review of the genotoxicity and carcinogenicity studies available to date on APM has been published previously (Belpoggi et al. 2006; Soffritti et al. 2005, 2006). In our opinion, the small number of animals used per sex and per group and the termination of these experiments after 110 weeks of age, rather than observing animals over their life span, represent limiting factors when evaluating the carcinogenic risk or safety of artificial sweeteners such as aspartame. It was for this reason, together with the growing use of APM in industrialized countries, that we designed and performed a mega-experiment using seven groups of Sprague-Dawley rats (100–150 per sex per group) treated with APM in feed at various dose levels (including one very close to the ADI for humans), from 8 weeks of age until natural death (Belpoggi et al. 2006; Soffritti et al. 2005, 2006). The study demonstrated for the first time that APM is a multipotential carcinogenic agent, capable of inducing, in our experimental conditions a) a significant, dose-related increased incidence of malignant tumor–bearing animals in males (p ≤ 0.05) and in females (p ≤ 0.01), particularly in females treated at 50,000 ppm (p ≤ 0.01); b) a significant dose-related increase in lymphomas/leukemias in both males (p ≤ 0.05) and females (p ≤ 0.01), particularly in females treated at doses of 100,000 (p ≤ 0.01), 50,000 (p ≤ 0.01), 10,000 (p ≤ 0.05), 2,000 (p ≤ 0.05), or 400 ppm (p ≤ 0.01); c) a significant, dose-related increased incidence (p ≤ 0.01) of transitional cell carcinomas of the renal pelvis and ureter and their precursors (dysplasias) in females treated at 100,000 (p ≤ 0.01), 50,000 (p ≤ 0.01), 10,000 (p ≤ 0.01), 2,000 (p ≤ 0.05), or 400 ppm (p ≤ 0.05); d ) a significant, dose-related increased incidence of malignant schwannomas of peripheral nerves (p ≤ 0.05) in males (Belpoggi et al. 2006; Soffritti et al. 2005, 2006). Given the consolidated experience of the European Ramazzini Foundation (ERF) in the conduct of long-term bioassays and the large number of rodents used in the study, the results attracted the attention of the scientific community, consumer and industry associations, and the national and international agencies responsible for food safety, including the Italian Superior Council of Health, the European Food Safety Authority (EFSA), the U.S. FDA, Health Canada, and others. At their request, we provided each of these agencies with all available raw data related to the study. To our knowledge, only the EFSA has issued an official opinion on our study, releasing on 5 May 2006 a 40-page report in which they concluded that it is not necessary to revise their previous opinion on the absolute safety of APM (EFSA 2006). Subsequent to our findings of hematopoietic cancers in rats, and in light of persistent concerns among the scientific community of an association between APM and brain cancers, Lim et al. (2006) published the results of a study that assessed the correlation between the consumption of aspartame-containing beverages and the incidence of these types of cancers. The findings were based on data derived from a prospective study conducted by the U.S. National Institutes of Health and the American Association of Retired Persons, using a cohort of > 285,000 men and > 188,000 women between 50 and 71 years of age, who had satisfactorily responded to a self-administered food frequency questionnaire. The questionnaire included questions on the consumption of beverages (soft drinks, fruit drinks, sweetened iced tea) potentially containing APM during the previous year. The questionnaires were mailed from 1995 to 1996 and the follow-up lasted until 2000. The conclusions of the study (Lim et al. 2006) did not support the hypothesis that APM increases hematopoietic or brain cancer risks. Recently a group of Italian authors (Gallus et al. 2007) published the results of an integrated network of case–control studies conducted in Italy between 1991 and 2004 on the potential correlation between artificial sweeteners (including APM) and cancer. The authors interviewed patients with histologically confirmed cancers of the oral cavity and pharynx (598), esophagus (304), colon (1,225), rectum (728), larynx (460), breast (2,569), ovary (1,031), prostate (1,294), and kidney (renal cell carcinoma 767). Controls were 7,028 patients (3,301 men and 3,727 women) admitted to the same hospitals for acute, nonneoplastic disorders. Cases and controls were interviewed during their hospital stay, using a questionnaire on subjects’ usual diet in the 2 years before diagnosis. The results reported a lack of association between artificial sweeteners and the risk of the aforementioned cancers. As soon as we perceived the carcinogenic effects of APM during the elaboration of the data in our first mega-experiment (Belpoggi et al. 2006; Soffritti et al. 2005, 2006), we planned an integrated program of long-term bioassays, beginning treatment from prenatal life, on > 4,000 rats and mice in order to better quantify the carcinogenic risks of aspartame. In this report we present the results of a second study on APM in which male and female Sprague-Dawley rats were exposed to very low doses of APM in feed (100 or 20 mg/kg bw) from fetal life until natural death.